It is important to comment on the physical phenomenon called scattering, the interaction of light with matter. When a beam of light falls on a particle of size , the characteristic dimension, the scattered energy will show changes with respect to intensity, directionality, wavelength, phase, and other properties of the wave. The property that shows the most pronounced change depends on the ratio of the wavelength and the particle diameter Broadly speaking, we have the following limits:

1. Ray Optics                                      

2. Wave Optics                                     

3. Quantum Optics                               

In the context of interferometry, schlieren and shadowgraph, the medium is taken be transparent and hence non-scattering. On the other hand, the medium contributes to wave propagation by altering the wave speed (the speed of light). The material property of relevance is the refractive index n defined as

Here, is the speed of light in vacuum and c is the speed of light in the transparent medium. It can be shown that the refractive index satisfies the inequality . The utility of refractive index in measurements arises from the fact that, for transparent media, it is a unique function of material density. Since density, in turn, will depend on temperature and species concentration, refractive index fields carry information related to heat and mass transfer processes.

For a medium that is partly absorbing and partly transparent, the refractive index is a complex quantity. The discussions in modules 4 and 5 deal with transparent media and images originate from the distribution of refractive index in the field of interest. The present module contains discussions on interferometry.